Humans and other mammals require daily intake of proteins for their life activities. Proteins are converted into amino acids in the digestive tract and utilized in vivo for growth, reproduction, assimilation, etc.
However, it is often difficult to take nutrients from ordinary foods under conditions such as postoperative lowering of function of the digestive tract and malnutrition due to apastia, starvation, etc., where nutrient supplementation is required. In addition, during or after hard physical labor or exercise, etc., metabolic function is enhanced and nitrogen is seriously lost by perspiration, and thus nutrient supplementation is necessary. In such cases, large quantities of proteins or amino acids must be taken in a readily absorbable form with minimized burden on the digestive tract.
Conventional methods for nutrient supplementation include intravenous injection and instillation of amino acids, and oral or intra-intestinal administration of nutrient compositions containing proteins of good quality such as meat extract, casein and albumen, amino acids, peptides, etc.
However, the use of some kinds of amino acids is limited because of their poor solubility, instability, etc. In particular, it is pointed out that supplementation of L-glutamine is necessary for a patient in postoperative catabolic condition since L-glutamine in the muscle of such a patient seriously decreases; but L-glutamine itself cannot be contained in known nutrient compositions which are sterilized by heating in the preparation process since it is unstable in solution and also has a poor stability to heat.
In order to supply L-glutamine which is unstable in solution as a nutrient composition for mammals, there have been developed nutrient compositions comprising L-glutamine in the form of .alpha.-L-aspartyl-L-glutamine (Japanese Published Unexamined Pat. Application No. 151156/1987), nutrient compositions comprising L-glutamine in the form of glycyl-L-glutamine (Japanese Published Unexamined Pat. Application No. 140923/1981), etc., by converting L-glutamine into a dipeptide.
Further, there is a report on the behavior of L-alanyl-L-glutamine in plasma and the uptake thereof by organs in Clinical Science, 75, 463 (1988).
In the nutrient compositions comprising .alpha.-L-aspartyl-L-glutamine or glycyl-L-glutamine, the stability of L-glutamine is considerably improved, but still they should be improved in many points such as stability under sterilizing conditions, absorption in vivo and utilization efficiency in vivo.
Further, when nutrient compositions comprising L-glutamine-containing dipeptides instead of L-glutamine are taken, these dipeptides must be hydrolyzed to amino acids in vivo. Conventionally used dipeptides were improved in stability of L-glutamine to heat, etc. but are not satisfactory in absorption in vivo and utilization efficiency in vivo.